BIOCOMPATIBLE Co-Cr-Mo ALLOY

ABSTRACT

The present invention is to provide a Co—Cr—Mo alloy excellent in mechanical properties such as yield strength and tensile strength. The present invention is a biocompatible Co—Cr—Mo alloy comprising, in mass %, more than 30% and not more than 36% of Cr, 5 to 8% of Mo, 0.20 to 0.65% of N, and the balance consisting of Co and inevitable impurities, and produced by layered manufacturing. The biocompatible Co—Cr—Mo alloy of the present invention preferably has a solidification structure of a dendrite structure and the primary arm spacing of the dendrite structure is not more than 5 μm.

TECHNICAL FIELD

The present invention relates to a biocompatible Co alloy, and particularly to a Co—Cr—Mo alloy excellent in mechanical properties, particularly, proof stress, tensile strength, and the like.

BACKGROUND ART

Co—Cr—Mo alloys have been used widely all over the world as a biocompatible material and, for example, a Co-28% Cr-6% Mo alloy (casting material) standardized as ASTM F75 has been known. However, the cast material standardized as ASTM F75 is difficult to sufficiently suppress solidification defects and segregation as it is and there is room for improvement of strength and ductility.

In order to solve such problems of the material standardized as ASTM F75, for example, Patent Document 1 proposes a casting material of a Co—Cr—Mo alloy with increased contents of Cr and nitrogen as compared with the above-mentioned standardized material.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: JP-A-2009-114477

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

According to the above-mentioned Patent Document 1, it is described that a cast Co alloy containing, in mass %, more than 30% and not more than 36% of Cr, 5 to 8% of Mo, and 0.20 to 0.65% of N has improved yield strength, tensile strength, and elongation as compared with the material standardized as ASTM F75.

In Patent Document 1, an ingot of a Co—Cr—Mo type alloy is produced by metal mold casting and mechanical properties thereof are measured. However, according to the investigations carried out by the inventors of the present invention, in the case where the Co—Cr—Mo type alloy described in Patent Document 1 is produced by sand mold casting which is employed most practically in biomaterials, it is supposed that the mechanical properties as disclosed in Cited Document 1 cannot necessarily be realized. Here, from the viewpoint of particularly high strength, it is generally effective to carry out hot processing such as hot rolling or hot forging. However, the Co—Cr—Mo alloy with the composition described in Patent Document 1 is poor in the hot processability and difficult to have high strength by hot processing and thus it is difficult to obtain a Co—Cr—Mo alloy excellent in yield strength, tensile strength and the like.

Accordingly, the present invention aims to provide a Co—Cr—Mo alloy excellent in mechanical properties such as yield strength (0.2% proof stress) and tensile strength.

Means for Solving the Problems

The present invention which has achieved the above-mentioned problem is a biocompatible Co—Cr—Mo alloy comprising, in mass %,

more than 30% and not more than 36% of Cr,

5 to 8% of Mo,

0.20 to 0.65% of N, and the balance consisting of Co and inevitable impurities, and

produced by layered manufacturing.

The biocompatible Co—Cr—Mo alloy of the present invention preferably has a solidification structure of a dendrite structure and the primary arm spacing of the dendrite structure is not more than 5 μm. The biocompatible Co—Cr—Mo alloy of the present invention has the 0.2% proof stress of not less than 700 MPa and the tensile strength of not less than 980 MPa, for example.

The present invention also comprises a powder to be used for producing the biocompatible Co—Cr—Mo alloy. The powder has a gist for having a particle diameter of not more than 100 μm.

The present invention also comprises a process for producing a Co—Cr—Mo alloy wherein the Co—Cr—Mo alloy having the above-described chemical composition is layered-manufactured.

In the production process of the present invention, layered manufacturing is preferably carried out by irradiating laser of an output power of not less than 50 W and adjusting the scan pitch in the plane direction to be not less than 0.1 mm. As well, the scan pitch in the plane direction means a irradiation spacing of laser.

Effects of the Invention

According to the present invention, a Co—Cr—Mo alloy excellent in 0.2% proof stress, tensile strength, and elongation is provided since a Co—Cr—Mo alloy with high Cr and high N is produced by layered manufacturing.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] FIG. 1 is a graph illustrating a relationship between the scan pitch in the plane direction and 0.2% proof stress.

[FIG. 2] FIG. 2 is a graph illustrating a relationship between the output power of laser, and tensile strength and elongation.

[FIG. 3] FIG. 3 is an optical microscope photograph obtained by observing the structure of a Co alloy produced in the Examples described later.

MODE FOR CARRYING OUT THE INVENTION

The inventors of the present invention repeatedly made investigations to provide a Co—Cr—Mo alloy excellent in mechanical properties such as yield strength and tensile strength and found that a Co—Cr—Mo alloy excellent in 0.2% proof stress, tensile strength and elongation is obtained by powdering a Co—Cr—Mo alloy with a composition described in Patent Document 1 and carrying out layered manufacturing of the powder, and the finding now lead to completion of the present invention. Hereinafter, the alloy composition of the present invention and layered manufacturing will be described in order.

A Co—Cr—Mo alloy of the present invention contains, in mass %, more than 30% and not more than 36% of Cr, 5 to 8% of Mo, and 0.20 to 0.65% of N.

Cr is an element indispensable for surely attaining corrosion resistance. In the present invention, the mechanical properties are further improved and the amount of dissolved N is increased by adjusting the Cr amount to be more than 30% (it means mass %, hereinafter the same in the chemical composition). The Cr amount is preferably not less than 31% and more preferably not less than 32%. On the other hand, if the Cr amount is excessive, the mechanical properties such as tensile strength and elongation are rather deteriorated. Therefore, the Cr amount is determined to be not more than 36% in the present invention. The Cr amount is preferably not more than 35% and more preferably not more than 34%.

Mo is an element effective for improvement of corrosion resistance and wear resistance. Therefore, the Mo amount is determined to be not less than 5%. The Mo amount is preferably not less than 6%. On the other hand, if the Mo amount is excessive, it results in deterioration of processability. Therefore, the Mo amount is determined to be not more than 8%. The Mo amount is preferably not more than 7%.

N is an element for forming a stable γ phase and having an action of improving ductility. N is an element also having an action of improving the 0.2% proof stress. Therefore, the N amount is determined to be not less than 0.20% in the present invention. The N amount is preferably not less than 0.25% and more preferably not less than 0.30%. On the other hand, if the N amount is excessive, Cr₂N is precipitated and mechanical properties are deteriorated. Therefore, the N amount is determined to be not more than 0.65%. The N amount is preferably not more than 0.60% and more preferably not more than 0.55%.

The composition of the Co alloy of the present invention is as described above and the balance is made up by substantially Co. Co is an element having corrosion resistance and wear resistance. In addition, it is naturally allowed that the Co alloy of the present invention contain inevitable impurities brought into depending on the raw materials, resources, the situation of production equipment, and the like. Further, in the present invention, if necessary, the Co alloy may contain at least one element selected from the group consisting of not more than 0.2% of C, not more than 1.00% of Ni, not more than 1.00% of Si, and not more than 1.00% of Mn.

The Co alloy of the present invention is characterized by being produced by layered manufacturing. The Co alloy produced by layered manufacturing has a fine solidification structure and in general, the solidification structure is a dendrite structure, and the primary arm spacing of the dendrite structure is, for example, not more than 5 μm and preferably not more than 1.5 μm. The lower limit of the arm spacing is generally around 0.5 μm.

Next, the layered manufacturing will be described. The layered manufacturing is described in, for example, Japanese Patent No. 4054075, and is a method for producing a compact by spreading a material powder in a layer form, melting and thereafter solidifying the material powder by irradiating the powder with electromagnetic radiation such as laser beam or corpuscular radiation such as electron beam. In the present invention, since the Co alloy with the above-mentioned composition is obtained by layered manufacturing, it is made possible to provide the Co alloy excellent in elongation in addition to 0.2% proof stress, tensile strength. Further, by layered manufacturing, the irradiation pattern of the radial rays is precisely controlled and therefore, shaping into a complicated shape is made possible.

According to the investigations made by the inventors of the present invention, there is a correlation between the mechanical properties (0.2% proof stress, tensile strength, and elongation) of a Co alloy and the condition of the layered manufacturing, and particularly the 0.2% proof stress is more improved by widening the scan pitch in the plane direction and particularly the tensile strength and elongation is more improved by increasing the output power of the radial rays. Concretely, the scan pitch in the plane direction is preferably adjusted to be not less than 0.1 mm, more preferably not less than 0.2 mm, and even more preferably not less than 0.3 mm. The upper limit of the scan pitch in the plane direction is, for example, not more than 0.5 mm for preventing pore formation. The output power of the radial rays is preferably not less than 50 W, more preferably not less than 100 W, and even more preferably not less than 150 W. Although it depends on the apparatus for outputting the radial rays, the upper limit of the output power of the radial rays is, for example, not more than 400 W.

Other production conditions for the layered manufacturing may be set properly and, for example, the radiation diameter (e.g., radius of circle) of radial rays may be adjusted to be about 0.1 to 1 mm; the distance from the radial ray source to the material powder may be adjusted to be about 100 to 10000 mm; and the layer thickness (thickness of one layer of the powder) may be adjusted to be about 0.01 to 0.1 mm. The atmosphere at the time of the layered manufacturing is also not particularly limited, and it is preferable to carry out the layered manufacturing in an atmosphere of an inert gas such as argon gas or nitrogen gas.

The material powder used for the layered manufacturing can be prepared by an atomization method (water atomization method or gas atomization method), a rotating electrode process, a ball mill method, and the like. It is preferable that the powder prepared by the above-mentioned method is sieved if necessary to have a particle diameter of not more than 100 μm (preferably not more than 70 μm, and more preferably not more than 50 μm). In addition, the particle diameter of the material powder is generally not below 5 μm. In this description, the particle diameter means the maximum diameter.

The type of radial rays in the layered manufacturing is not particularly limited, but laser of electromagnetic radiation is employed in the present invention. Examples of the type of laser include YAG laser, excimer laser, semiconductor laser, and CO₂ laser.

The Co alloy of the present invention produced by layered-manufacturing the Co alloy with the above-mentioned composition is excellent in mechanical properties such as 0.2% proof stress, tensile strength, and elongation. The 0.2% proof stress is, for example, not less than 700 MPa (preferably not less than 730 MPa and more preferably not less than 7 60 MPa) and the tensile strength is, for example, not less than 980 MPa (preferably not less than 1000 MPa and more preferably not less than 1020 MPa). The elongation (total elongation) is, for example, not less than 8.0% (preferably not less than 10.0% and more preferably not less than 15.0%).

The structure of the Co alloy of the present invention is generally a dendrite structure. Because of the effect of the anisotropy of the structure, the mechanical properties of the Co alloy are different depending on the directions (directions to the layer direction). Concretely, the 0.2% proof stress and tensile strength show higher values in the direction perpendicular to the layered direction and the elongation shows a higher value in the direction parallel to the layered direction. Consequently, it is preferable to produce a biocompatible material while properly adjusting the direction to which a load is mainly applied at the time of use of the material and the layered direction, in accordance with the use application of the biocompatible material. Concretely, in the case of a use application in which the 0.2% proof stress and tensile strength are mainly required, it is proper to produce the Co alloy of the present invention by layered manufacturing in a manner that the load direction at the time of use of the biocompatible material and the layered direction are perpendicular to each other. Also, in the case of a use application in which elongation is mainly required, it is proper to produce the Co alloy of the present invention by layered manufacturing in a manner that the load direction at the time of use of the biocompatible material and the layered direction are parallel to each other.

This application claims the benefit of priority based on Japanese Patent Application No. 2011-231703 filed on Oct. 21, 2011. All the contents of Japanese Patent Application No. 2011-231703 filed on Oct. 21, 2011 are incorporated herein by reference.

EXAMPLES

Hereinafter, the present invention will be described more concretely with reference to examples. The present invention is not limited to the following examples and various modifications can be appropriately made without departing from the gist of the invention as defined above or hereinafter, all of which fall within the technical scope of the present invention.

A molten Co alloy with the chemical composition described in Table 1 was prepared and a Co alloy powder was produced by a water atomization method. Thereafter, the powder was sieved to produce a Co alloy powder with a particle diameter of not more than 45 μm.

Each Co alloy in form of a dumbbell type tensile test specimen according to JIS T6115, the dentistry casting cobalt-chromium alloy, was produced from the above-mentioned powder by a layered manufacturing apparatus (EOSINT M250 xtended) with the laser output power and scan pitch (in plane direction) as described in Table 1 (test Nos. 1 to 7, 9 and 11 to 15). Test Nos. 1 to 5, 9, and 11 to 14 were layered-manufactured in a manner that the tensile test direction and the layered direction were to be parallel to each other and test Nos. 6, 7, and 15 were layered-manufactured in a manner that the tensile test direction and the layered direction were to be perpendicular to each other. The layered manufacturing was carried out in an argon atmosphere, the irradiation diameter of laser was 0.4mm (400 microns), and the layered thickness was 0.05 mm.

Each specimen was subjected to measurement of 0.2% proof stress, tensile strength, and elongation (total elongation) according to JIS T6115. The number of tests for each test No. was 3.

For comparison, Table 1 also shows the results of a specimen (test No. 8) produced from Co-33 mass% Cr-5 mass % Mo-0.34 mass % N satisfying the composition of the present invention by a centrifugal casting method using a room temperature sand mold and a specimen (test No. 10) produced from Co-29 mass% Cr-6 mass % Mo, a material standardized as ASTM F75, by a centrifugal casting method using a room temperature sand mold. In Table 1, in order to describe that the samples were produced by the centrifugal casting method, “As-Cast” is written in the column of laser output power.

TABLE 1 Output power of 0.2% proof Tensile Elongation Test Compositon of alloy laser Scan pitch^(1) stress strength El No. (mass %) (W) (mm) Tensile direction^(2) (MPa) (MPa) (%) 1 Co—33Cr—5Mo—0.34N 100 0.1 parallel 789 1017 15.9 2 150 0.1 parallel 760 1031 18.8 3 200 0.1 parallel 744 1040 22.4 4 200 0.2 parallel 814 1030 15.9 5 200 0.3 parallel 829 1050 16.7 6 200 0.1 perpendicular 900 1195 11.7 7 200 0.2 perpendicular 863 1123 8.6 8 As-Cast — — 606 910 14.2 9 Co—29Cr—6Mo 200 0.1 parallel 538 949 16.4 10 As-Cast — — 223 500 3.2 11 Co—33Cr—5Mo—0.41N  65 0.1 parallel 862 1117 21.8 12  98 0.1 parallel 845 1123 24.8 13 130 0.1 parallel 819 1120 26.2 14 130 0.2 parallel 856 1110 18.3 15 130 0.1 perpendicular 996 1328 13.2 ^(1)It means the scan pitch in the plane direction. ^(2)It measns the tensile direction to the layered direction.

According to Table 1, all specimens of test Nos. 1 to 7 and 11 to 15, which are Co alloys of the present invention, were excellent in mechanical properties of 0.2% proof stress, tensile strength, and elongation. Further, from the results of these tests, the effects on the mechanical properties given by the scan pitch and the output power of the radial rays were made clear. FIG. 1 is a graph illustrating a relationship between the scan pitch in the plane direction and 0.2% proof stress, and FIG. 2 is a graph illustrating a relationship between the output power of the radial rays (output power of laser in the examples) and tensile strength and elongation. According to FIG. 1 and FIG. 2, it can be understood that the 0.2% proof stress is improved as the scan pitch becomes wider and that the tensile strength and elongation are improved as the output power of laser becomes higher.

Comparing test No. 3 with test No. 6, it can be understood that elongation is excellent in the case where the tensile direction is parallel to the layered direction and that the 0.2% proof stress and tensile strength are excellent in the case where the tensile direction is perpendicular to the layered direction. The same tendency can be seen in the comparison of test No. 4 with test No. 7 or of test No. 13 with test No. 15.

Further, FIG. 3 shows a photograph of a structure of the specimen of test No. 3 observed by an optical microscope. FIG. 3( a) is a photograph of a structure of a cross section perpendicular to the layered direction, and FIG. 3( b) is a photograph of a structure of a cross section parallel to the layered direction. According to FIG. 3, it can be understood that a fine dendrite structure is formed.

When the arm spacing of primary arms of the dendrite structure for the test No. 3 was measured by measuring the number n of times the dendrite interface crossed a certain reference length L and carrying out calculation according to L/(n−1), it was 1.5 μm. The arm spacings of primary arms of the dendrite structure for the test Nos. 1, 2, 4 to 7, and 11 to 15 were all not more than 5 μm, but the arm spacings of primary arms of the dendrite structure for the test Nos. 8 and 10, which are out of the scope of the present invention, exceeded 5 μm because the specimens were produced by a centrifugal casting method.

INDUSTRIAL APPLICABILITY

The Co—Cr—Mo alloy of the present invention can be used suitably as an biocompatible material, for example, for dentistry, orthopedics, and the like. 

1. A biocompatible Co—Cr—Mo alloy comprising, in mass %, more than 30% and not more than 36% of Cr, 5 to 8% of Mo, 0.20 to 0.65% of N, and the balance consisting of Co and inevitable impurities, and produced by layered manufacturing.
 2. The biocompatible Co—Cr—Mo alloy according to claim 1, wherein a solidification structure is a dendrite structure, and the primary arm spacing of the dendrite structure is not more than 5 μm.
 3. The biocompatible Co—Cr—Mo alloy according to claim 1, wherein the 0.2% proof stress is not less than 700 MPa and the tensile strength is not less than 980 MPa.
 4. A biocompatible Co—Cr—Mo alloy powder used for producing the biocompatible Co—Cr—Mo alloy according to claim 1, having a particle diameter of not more than 100 μm.
 5. A process for producing an biocompatible Co—Cr—Mo alloy, wherein the Co—Cr—Mo alloy powder according to claim 4 is layered-manufactured.
 6. The process for producing an biocompatible Co—Cr—Mo alloy according to claim 5, wherein the layered manufacturing is carried out by irradiating laser of an output power of not less than 50 W and adjusting the scan pitch in the plane direction to be not less than 0.1 mm. 